1. Field of the Invention
The present invention relates to an optical arrangement of an infrared camera for detecting an infrared ray and converting the infrared rays to an image signal.
2. Background Art
An infrared camera detecting a radiant energy of far-infrared, mid-infrared, near-infrared and converting the radiant energy to an image is used for, for example, detecting a person and an animal in the dark for security monitoring, car driving or disaster. As an imaging method for an infrared camera, for example, a thermal-type infrared image sensor which thermally detects an infrared ray is used as an imaging unit as disclosed in Japanese Patent Laid-Open No. 10-281864. The thermal-type infrared camera disclosed in Japanese Patent Laid-Open No. 10-281864 includes an imaging unit is made by thermal coupling a temperature detection unit whose physical properties change according to the temperature and an infrared absorbing layer which generates heat according to the amount of an entered infrared rays. When the infrared absorbing layer generates heat by the entered infrared ray, the heat elevates the temperature of the temperature detection unit and change physical properties of the temperature detection unit which can be detected as a current or volt change, and then output a current or volt change as an electric signal for an amount of the infrared ray.
Conventional infrared cameras such as a security camera for fixed-point monitoring and a night-vision camera for an automobile are capable of wide-angle imaging, but can just judge the presence or absence of people and/or animals, i.e. the definition is poor. However, for lenses used in an infrared camera, high definition is required and anticipation for interchangeable lenses that can be selectively used for the purposes, wide-angle or high-magnification, and other intended purposes almost same with a visible light camera depending on requirement is increasing in recent years.
For example, Japanese Patent Laid-Open No. 2009-63942 discloses an imaging apparatus including a lens unit for a far-infrared camera lens having a plurality of lenses designed to be wide angle and an infrared detector. A lens for the infrared camera is made of germanium which can transmit an infrared ray. These lenses for infrared cameras have a high correlation between the temperature and the refractive index of the lens material, and hence even a slight change in temperature tends to change refractive index of the lens material to result defocusing. Further, the infrared detector is affected by the surrounding temperature change. So, in the infrared cameras, such a measure in which a shutter which opens and closes in a predetermined interval is provided between the lens and the detector (two-dimensional detector); the temperature of the two-dimensional detector is corrected in a state in which the shutter is closed; an output is obtained from the two-dimensional detector immediately just after the shutter opens; and thereby the reduction in imaging accuracy due to temperature change around the two-dimensional detector is prevented as in an infrared ray imaging system disclosed in Japanese Patent Laid-Open No. 2010-261801.
In the infrared cameras for vehicle-mounting or mobile use, improved imaging accuracy and downsizing are required. However, in the infrared camera detecting amount of the infrared rays as a heat, an infrared detector should be disposed in a vacuum chamber or a shutter should be provided for temperature control as described above. That is, increasing in size of the apparatus was inevitable to improve the imaging accuracy.
Then, an object of the present invention is to provide an optical arrangement of an infrared camera having both an improved imaging accuracy and a reduced size of the apparatus.